(1) Field of the Invention
The present invention relates to room temperature or thermally polymerizable/curable episulfide based compositions for making polymerized/cured episulfide based resins, and in particular episulfide based compositions which can be fastly polymerized/cured, as well as to a room temperature or thermal polymerization process for making such episulfide based resins.
The polymerizable compositions and the polymerization process of the invention are particularly useful for making optical articles such as lenses, prisms, optical fibers, filters or for making different types of substrates, such as information recording substrates.
The polymerizable compositions and the polymerization process of the invention are also specifically suited for making various coatings and in particular coatings for optical applications.
(2) Description of the Prior Art
Plastic materials are widely used in the optical field and particularly in the ophthalmic field for their lightness, high impact resistance and tintable capability by immersion in a bath containing an organic dye.
Optically transparent plastic materials having a high refractive index nD=1.6 or even higher have been developped recently which renders it possible to manufacture optical articles such as lenses of lower thickness for an equivalent corrective power (optical power).
A class of such materials is comprised of episulfide based resins.
Document U.S. Pat. No. 5,807,975 discloses thermal polymerization and curing of diepisulfides, in particular for the manufacture of optical materials such as a lens material for spectacles. This thermal polymerization/curing of diepisulfides lasts for several hours, generally about 20 hours.
Episulfide compounds and their polymerization process are disclosed in EP-A-761.665 and EP-A-785.194.
Unfortunately, the polymerization of these polymerizable compositions needs a long thermal cycle, generally between 8 hours to several days in order to obtain fully polymerized articles without striation or strain.
Others have disclosed thermal or room temperature polymerization of episulfide compounds using, as catalyst, a salt of formula:Mmp+Yn−
in which Mp+ is a cation selected from the group consisting of alkaline metals, alkaline earth metals, transition metals and ammonium groups, Y− is an anion of an acid fulfilling the condition 0.5≦Pka≦14, p is the valency of the cation and n=m×p. A preferred catalyst is KSCN.
Although the polymerization time is very significantly decreased when using such a salt catalyst, there is still a requirement to speed up the polymerization of any kind of episulfides without inducing striation and yellowness or without giving a run-away reaction.